Asus ENGTX670 DirectCU II Edition

Considering the Direct CUII cards are typically overclocked due to the superb cooling I would go with at least 550 watts and would be much more comfortable with 600 or 650Watt PSU. Even quality models of this wattage range are not too expensive.

I would recommend this one:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817139020

$89 in price
$10 off with promo code EMCNCJC32 (ends July 23rd)
Plus $20 rebate = $59.

The rebate is via a rebate card but still the end value is $59

I rather have a little too much power than to try to get by with the lowest amount of power possible and run the risk of unstable currents.